1. Unit Two: Membrane Physiology, Nerve, and Muscle
Chapter 5: Membrane Potentials and Action Potentials
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Basic Physics of Membrane Potentials
Membrane Potentials Caused by Diffusion
“Diffusion potential” caused by an ion
concentration difference
Relation of the diffusion potential to the
concentration difference-Nernst Potential
Fig. 5.1

3. Basic Physics (cont.)
Calculation of the diffusion potential when the membrane
is permeable to several different ions.

4. Measuring the Membrane Potential
Fig Measurement of the membrane potential of a nerve fiber using
a microelectrode.

5. Resting Membrane Potential
The Sodium-Potassium Pump: active transport
of ions through the membrane

6. Membrane Potential (cont.)
Leakage of Potassium Through the Nerve
Membrane
Fig The Na-K pump and the
Na leak channels

7. Origins of the Resting Membrane Potential
Origin of the Normal Resting Membrane Potential
Contribution of the potassium diffusion potential
Contribution of the sodium diffusion through
the nerve membrane
c. Contribution of the sodium-potassium pump

8. Fig 5.5 A. Membrane potential
caused by K alone
B. Membrane potential
caused by both Na
and K
C. Membrane potential
and K and the Na-K
pump

9. Nerve Action Potential
Fig Typical action potential

10. Nerve Action Potential (cont.)
Resting Stage-polarized because of the -90 mV
membrane potential
Depolarization Stage-suddenly permeable to Na
with large numbers of ions diffusing into the cell
Repolarization-the sodium channels close and the
potassium channels open

11. Nerve Action Potential (cont.)
Voltage-gated Sodium Channel—activation and
inactivation of the channel
Fig. 5.7

12. Nerve AP (cont.)
Activation of the sodium channel-when the membrane potential
approaches -70 to -50 mV, the activation gates undergo a
conformational change and open
The sodium permeability may increase from fold
Inactivation of the sodium channel-the same increase in voltage
that opens the activation channel closes the inactivation channel
It closes slower than the activation gate opens. The inactivation
gate generally will not reopen until the membrane potential
returns to or near the original resting potential

13. Voltage Gated Potassium Channels and Their Activation
Nerve AP (cont.)
Voltage Gated Potassium Channels and Their
Activation
During the resting state, the K+ channels are closed
As the potential approaches 0 mV, K+ diffusion begins
There is a slight delay so they open just as the Na+
channels close
Fig. 5.9

14. Nerve AP (cont.)
Summary
Fig. 5.10

15. Initiation of the AP
Positive feedback cycle opens the Na+ channels-
if any event causes a rise in voltage will open
Na+ activated channels, which causes more and
more Na+ channels to open; this then initiates
an AP
Threshold for the initiation of the AP-usually a
sudden rise in voltage of mV is required

16. Propagation of the AP Direction of the AP All or None Principle
Fig Propagation of the AP

17. Cardiac Muscle
Plateau in the AP
The membrane does not repolarize immediately
The potential remains on a plateau
Plateau prolongs the period of repolarization
Plateau is caused by two types of channels
Usual voltage activated Na+ channels, called
fast channels
Voltage activated Na+-Ca++ channels, slow
channels
Voltage gated K+ channels are slower than usual to
open

18. Fig. 5.13 Action potential in mV from a Purkinje fiber in the heart

19. Cardiac Muscle (cont.)
Rhythmicity of Excitable Tissues-Repetitive
Discharge
Repetitive self induced discharges can occur
in the heart (rhythmic beat), intestine,
(peristalsis), and rhythmic control of
breathing

20. Cardiac Muscle (cont.)
Re-excitation Process Necessary for Spontaneous
Rhythmicity-the following sequence occurs
Some Na+ and Ca++ ions flow inward
This increases the membrane voltage in the positive
direction, which further increases permeability
Still more ions flow inward
The permeability increases until an AP is generated

21. Fig Rhythmic action potentials similar to those recorded in the rhythmic control center of the heart

22. Special Characteristics of Signal Transmission
Myelinated and Unmyelinated Nerve Fibers
Saltatory vs Continuous Conduction
Summation of Sub-threshold Potentials
Absolute and Relative Refractory Periods

23. Fig. 5.15 Cross-section of a small nerve trunk containing
both myelinated and unmyelinated fibers

24. Fig. 5.16

25. Fig. 5.17 Saltatory conduction along a myelinated fiber